From: Nigel Cunningham <nigel@suspend2.net>
Organization: Suspend2.net
To: "Rafael J. Wysocki" <rjw@sisk.pl>
Subject: Re: [ 01/10] [Suspend2] kernel/power/modules.h
Date: Fri, 3 Feb 2006 07:27:43 +1000
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Cc: Pavel Machek <pavel@ucw.cz>, Pekka Enberg <penberg@cs.helsinki.fi>,
       linux-kernel@vger.kernel.org
References: <20060201113710.6320.68289.stgit@localhost.localdomain> <20060202093859.GA1884@elf.ucw.cz> <200602021353.30802.rjw@sisk.pl>
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Hi.

On Thursday 02 February 2006 22:53, Rafael J. Wysocki wrote:
> Hi,
>
> On Thursday 02 February 2006 10:38, Pavel Machek wrote:
> > > Its limitation , however, is that it requires a lot of memory for the
> > > system memory snapshot which may be impractical for systems with
> > > limited RAM, and that's where your solution may be required.
> >
> > Actually, suspend2 has similar limitation. It still needs half a
> > memory free, but it does not count caches into that as it can save
> > them separately.
>
> I didn't know that.  [If that is the case, I wonder what Nigel means by
> the "whole memory image".  Nigel?]

The LRU almost always easily accounts for more 50% of memory in use. Suspen=
d2=20
writes LRU pages to disk, then uses those pages to store the atomic copy of=
=20
the remainder of memory. That's how I overcome the 50% problem and still=20
really do get a full image of memory. If the LRU is smaller than the=20
remainder of memory in use, we allocate extra memory if possible. If that=20
still doesn't give enough memory for the atomic copy, we seek to free memor=
y=20
until that constraint is satisfied. If we free everything we can, and still=
=20
can't satisfy that constraint, we give up and return control to the user. I=
n=20
99% of the cases, however, no freeing of memory is required and the user=20
really can get a full image of memory saved.

> > That means that on certain small systems (32MB RAM?), suspend2 is going
> > to have big advantage of responsivity after resume. But on the systems
> > where [u]swsusp can't suspend (6MB RAM?), suspend2 is not going to be
> > able to suspend, either. [Roughly; due to bugs and implementation
> > differences there may be some system size where one works and second one
> > does not, but they are pretty similar]
>
> Generally speaking, my perception is that suspend2 may be preferrable bel=
ow
> 256 MB of RAM.  Moreover there are some people who seem to prefer
> entirely kernel-based suspend, and I'm not going to develop the code
> in swap.c and disk.c any further (of course with the exception of bugfixes
> etc.).  Nigel has done it already so perhaps there is a room for his code,
> too, _provided_ _that_ it is accepted.

All of the machines I regularly use have 512M+ of memory. Suspend2 is=20
definitely preferable on them because I've worked hard to maximise I/O=20
throughput. Last time I measured swsusp throughput, it was 16MB/s on my old=
=20
Omnibook. Suspend2 achieved ~25MB writing and 50MB/s reading when using LZF=
=20
compression (933 Celeron), or the 35MB/s uncompressed (the maximum throughp=
ut=20
that could be achieved according to tools like hdparm -t) on the same syste=
m.=20
With the same drive in my new laptop (amd64 M34), I get ~70MB/s read/write=
=20
(depending on cpufreq settings). My 3G P4s at work and home do about 70
(w)/110(r) MB/s. (All of this is using LZF compression).

Regards,

Nigel

> Greetings,
> Rafael
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=2D-=20
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